<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .demo a{
            float: left;
            display: block;
            height: 50px;
            width: 50px;
            border-radius: 10px;
            background: blue;
            text-align: center;
            color: gainsboro;
            text-decoration: none;
            margin-right: 5px;
            font: bold 20px/50px Arial;
        }

        /*属性名，属性名 = 属性值（正则）
        = 绝对等于
        *= 包含这个元素
        ^= 以这个开头
        $= 以这个结尾
        */

        /*存在id属性的元素：a[]{}*/
        /*a[id]{!*所有带id的a标签*!
            background: yellow;
        }*/
        /*id=first的元素*/
        /*a[id=first]{
            background: blue;
        }*/
        /*class中有links的元素*/
        /*a[class*=links]{
            background: yellow;
        }*/

        /*选中href中以http开头的元素*/
        /*a[href^=http]{
            background: yellow;
        }*/
        /*选中href中以pdf结尾的*/
        a[href$=pdf]{
            background: yellow;
        }

    </style>
</head>
<body>

<p class="demo">
    <a href="http://baidu.com" class="links item first" id="first">1</a>
    <a href="" class="links item active" target="_blank" title="test">2</a>
    <a href="images/123.html" class="links item">3</a>
    <a href="images/123.png" class="links item">4</a>
    <a href="images/123.jpg" class="links item">5</a>
    <a href="abc" class="links item">6</a>
    <a href="/a.pdf" class="links item">7</a>
    <a href="/abc.pdf" class="links item">8</a>
    <a href="abc.doc" class="links item">9</a>
    <a href="abcd.doc" class="links item last">10</a>
</p>
</body>
</html>